We have continued our studies of cerebrospinal fluid (CSF) levels of various hormones, peptides, and monoamine metabolites in rhesus monkeys: (1) Studies with prolactin have demonstrated a significant diurnal rhythm in CSF as well as a blood-CSF gradient; a ventricular-lumbar gradient within the CSF is not present under baseline conditions but develops following treatment with thyrotropin stimulating hormone, which stimulates prolactin release; (2) an investigation of CSF serotonin and melatonin concentrations sampled over a 24-hour period revealed a marked diurnal rhythm in serotonin which was nearly identical in timing to the melatonin rhythm; (3) studies of CSF cholecystokinin have shown no response to intravenous amphetamine administration. These investigations are beginning to clarify the potential effects of some neuroendocrine substances on the brain.